1. Technical Field
In a principal aspect, the present invention generally relates to a method of soil densification and improvement for the purpose of forming a stiffened support pier in a cavity within the densified and improved soil.
The present invention additionally relates generally to the field of civil and construction engineering and, more specifically, is directed to methods and apparatus for providing load supporting aggregate piers in the earth capable of supporting a multitude of possible structures including, but not limited to, buildings, roads, bridges and the like.
2. Background Art
Many soils are deficient in their capability to incorporate a shallow support system such as shallow foundations or a shallow mat system. Consequently, when building a structure, highway embankment or retaining wall, it is often necessary to provide a special foundation support for the structure and various techniques have been developed to provide adequate subsoil support for such structures to prevent excessive settlements and to prevent bearing failures. For example, pilings may be driven into the ground to bedrock. Various techniques have also been developed for densifying and improving the ground and utilizing the improved ground in combination with pilings or stiffened piers or footings constructed therein.
It has been conventional practice for many years to provide vertical, elongated cavities in the earth for receiving aggregate to form what is known as “stone columns”. In one conventional procedure cavities are formed by vertically vibrating a vibroflot cylindrical tube into the ground. The vibroflot tube has motor driven eccentric weights in its lower end for applying lateral or radial vibrations to the tube and the short conical tool. Penetration of the earth by the tube is assisted by either air or water jetting means. Older devices of the foregoing type use water jetting means and drop aggregate, crushed stone or other granular materials into the cavity from the ground surface in what is referred to as a “wet method”. More recent variations have employed air jetting and introduction of stone through the tube.
Major problems with the wet method process are that it adds water to the cohesive clay soils around the vibroflot so as to soften the soil, and it produces effluent containing suspended particles that is often required to be treated. Unfortunately, the application of horizontal vibration applied to the stone results in a column having low stiffness in comparison to short aggregate piers as discussed in the following paragraphs.
A more recently employed method of providing short aggregate piers is that of Fox et al. U.S. Pat. No. 5,249,892, which teaches use of a rotary drill to form a cavity typically of 18 to 36 inches in diameter, in the manner discussed in column 5, of the patent. Upon completion of the cavity, a thin lift (layer) of aggregate is placed in the bottom of the cavity and compacted vertically and outwardly by high energy impact devices (hydraulic hammers) applying direct downward and high frequency ramming to each thin lift of stone with the procedure then being repeated with subsequent thin stone lifts until the cavity is filled to complete the short pier. Shortcomings of such procedures include the required use of a casing to stabilize the sidewalls of the cavity above its lower end, when installations are in unstable soils which cave in, such as sands and sandy silts. Also, instability at the bottom of the cavity in granular soils with a high groundwater level is a frequent problem because of the water attempting to flow or pipe into the casing so as to create unstable conditions at the bottom of the cavity. Moreover, the depth of the cavity is limited to approximately 30 feet because of structural limitations of the equipment. A further problem arises in soft, cohesive or organic soils in which the load capacity of the pier to support loads is limited by the fact that the soft soil provides limited resistance to outward bulging movement of the stone piers.
Fox U.S. Pat. No. 6,354,766 discloses a variety of special techniques, including pre-loading, chemical treatment and use of mesh reinforcement procedures to enhance the construction and test the properties of short aggregate piers.
Fox U.S. Pat. No. 6,354,768 discloses the use of expandable bladders for densifying soil adjacent or below stone piers.
Another method of forming a stone pier is disclosed in U.S. Pat. No. 6,425,713 in which a lateral displacement pier, also know as a “cyclone pier”, is constructed by driving a pipe into the ground, drilling out the soil inside the pipe and filling the pipe with aggregate. The pipe is then used to compact aggregate in thin lifts by use of a beveled edge at the bottom of the pier for compaction. Piers fortified by this method can be installed to great depths such as 50 feet and in granular soils. Limitations of this approach include the need for a heavy crane for installation and a drill rig to drill out the casing. Additionally, the system is cumbersome and slow to install when the installation uses a normal crane and pipe having diameters such as listed in the patent.
Another system developed by Mobius and Huesker in Germany provides an encased stone column by pushing a closed-ended pipe into soft ground by use of a vibratory pile driving hammer mounted at the top of the pipe. When the lower end of the pipe reaches designed depth, a geotextile sock or bag is inserted into the inside of the pipe. This sock is then filled with crushed stone poured from the ground surface. After the sock is filled a trap-door opens at the bottom of the pipe and the pipe is extracted upwardly while the geotextile sock and its contents remain in the excavation. The primary advantage of this system is that the geotextile sock prevents the bulging of the crushed stone into the surrounding soil when loaded. However, a number of disadvantages include the fact that the column is not compacted and does not have high stiffness sufficient for supporting buildings and the like. Additionally, this system must be installed in very soft or loose soil that can be penetrated by closed-ended pipe pile driven with a vibratory pile driving hammer.
Another prior system developed by Nathaniel S. Fox employs a 14 inch to 16 inch diameter tamper head attached to the lower end of an 8 inch to 10 inch diameter cylindrical pipe. The pipe is vibrated into the ground and is filled with crushed stone once the tamper head is driven to the desired designed depth. The tamper head is then lifted to allow stone to fall into the cavity following which the tamper head is driven back downwardly onto the stone for densifying the stone.
A deep dynamic compaction system developed by Louis Menard employs a heavy weight which is dropped from a great height to pound the ground. Each drop creates a crater at the ground surface and generates significant ground shaking and causes granular soils to densify for the future support of structures. The system can be employed by placing fresh stone in the cavities formed by the dropped weight and then tapping the stone downward to form stone pillars used to support vertical loads. Similar methods are illustrated in United Kingdom Patent No. 369,816, Italian Patent No. 565,012, and French Patent No. 616,470. The disadvantages of these processes include the need for a large crane to lift the dropped weight and the excessive vibration that is induced during tamping.
Another system for making aggregate piers, involving driving a pointed mandrel has been used by a contractor in the United Kingdom and is disclosed in a brochure of Roger Bullivant Ltd dated June 2002. The disclosed device uses a vibrator piling hammer to direct the mandrel into the ground to provide a cavity for receipt of crushed stone. The mandrel has a sharply pointed end, which inhibits the compaction of the stone at the top of the pier.
Densification of the soil and construction of a stiffened pier column using the techniques of the type described in the aforesaid prior art comprises a mechanical densification process. Various mechanical means are utilized to alter, densify and otherwise improve the characteristics of the soil enabling the soil to effectively incorporate support piers. The process also produces a stiffened pier, which in combination with the improved adjacent soil, results in an effective structural support system for shallow foundations, slabs and mats.
A problem typically arises in sandy soil and other unstable soils in that drilled holes often cave in and require expensive preventive measures to prevent the cave-ins. Another problem with drilled holes is that cuttings are brought to the ground surface and they require disposal. This later problem is particularly onerous when the soils being penetrated are contaminated, since disposal of contaminated soils is extremely expensive.